Conventionally, a magnet for an NMR analyzer consists of co axially nested, multi layer solenoid coils 4, as shown in FIG. 9, in which the magnetic axis is vertically oriented.
An access port 21 vertically passes through the magnet in the vicinity of the magnetic axis so that a sample can be inserted. A measurement sample is inserted from above and a probe 10 containing an antenna (coil) for detecting signals is inserted from below. A conventional NMR analyzer uses a saddle-type or bird-cage-type antenna (coil).
It is possible to improve detection sensitivity of the NMR signal by arranging the shapes of the sample and antenna (coil) and their position al relationships.
FIG. 2 shows the position al relationships among a sample, detection coil and the magnetic field. “Document about NMR” by Yogi Arab, published by Marten, 2000, p.326 describes how the shape of the detection coil improves sensitivity. According to the document, by disposing a solenoid-type detection coil 5 having a coil axis (virtual axis) perpendicular to the main magnetic field generated by a magnet and disposing a sample 11 so that it passes through the coil 5, sensitivity can be increased at least 1.4 times more than the sensitivity when only coil shape is considered.
However, in conventional NMR analyzers, the configuration in which a sample is disposed perpendicularly to the direction of the main magnetic field to enable solenoid-type detection is not common and impossible except for a special purpose, for example, the use of a micro probe in which a solenoid-type detection coil is directly wound around an extremely small test tube containing a sample.
Japanese laid-open Patent Publication No. Hie 07-333311 discloses an example of a sample insertion method and the shape of a magnet which enables a sample to be positioned and the detection coil to be configured as mentioned above. The document describes a method in which a short-length sample 11 is mounted to a probe so that it is perpendicular to the magnetic axis, and the sample and the probe are together inserted into the magnet.
In this method, every time the sample is mounted and removed, the probe also has to be retrieved and inserted. Thus, operability is low, and also, there is the possibility that the probe will need to be adjusted every time it is retrieved and inserted. Thus, this method is not desirable.
Furthermore, because the length of the sample used is limited, it is difficult to embody a flow-type NMR analyzer in which samples are continuously and externally provided and moved through a sample injector. Also, replacement of a sample only, as disclosed in the document, is impossible in those cases where a solenoid-type antenna is used.
Here is another example. Although patent document 2 does not mention a method that improves sensitivity by using a solenoid-type antenna, it describes a split-type magnet. By using the described split-type magnet having an insertion port located in the magnet's gap, it is theoretically possible to dispose a sample, which passes through a solenoid-type antenna, at the center of the magnetic field so that the sample is perpendicular to the main magnetic field.
Japanese laid-open patent Publication No. Hie 07-240310 describes a method to dispose a sample and a probe in the magnet's gap. However, since the split-type magnet must have a gap, to obtain maximum strength in the magnetic field, electric current cannot flow through the most effective area. Accordingly, to generate the necessary strength in the central magnetic field, more current must flow through this magnet than the amount of current that flows through a magnet without a gap. Furthermore, to obtain necessary uniformity of the magnetic field, configuration of the coil is limited.
As a result, the diameter of the split-type magnet increases and the generated electromagnetic force also increases. In addition, to obtain necessary uniformity of the magnetic field, a coil that generates a magnetic field in the reverse direction of the main magnetic field must be installed. Thus, it is very difficulty to create such a magnet.